The invention relates generally to methods and kits for measuring von Willebrand factor (VWF), and more particularly to methods and kits for measuring VWF that do not require a platelet aggregation agonist, such as ristocetin.
VWF is a multimeric glycoprotein synthesized by megakaryocytes and endothelial cells, which is subsequently secreted into blood plasma as a spectrum of multimers. VWF binds other proteins, especially proteins involved in hemostasis, such as Factor VIII (an essential clotting factor that participates in the intrinsic pathway of blood coagulation) and platelet glycoprotein Ib (GPIb; a component of a platelet adhesion receptor complex). VWF is deficient or defective in von Willebrand disease (VWD) and is involved in a large number of other diseases, including thrombotic thrombocytopenic purpura, Heyde's syndrome and possibly hemolytic-uremic syndrome. See, Sadler J, “Biochemistry and genetics of von Willebrand factor”. Annu. Rev. Biochem. 67:395-424 (1998). VWF levels can be affected by many factors including ABO blood group and ethnicity.
VWD is a common bleeding disorder characterized by either qualitative or quantitative defects in tests for VWF. Symptoms of VWD include easy bruising, menorrhagia and epistaxis. Currently, many types of hereditary VWD are known (e.g., type 1; type 2A, 2B, 2M, 2N and type 3, as well as platelet-type, pseudo VWD, which results from a defect in platelet GPIb); however, acquired forms of VWD are also known, but are less frequently observed. Of particular interest herein is platelet-type, pseudo VWD. In contrast to the other forms of VWD, the genetic defect in platelet-type, pseudo VWD is in platelets rather than VWF and is characterized by abnormally high binding affinity of an individual's platelets to VWF, leading to a characteristic platelet hyper-responsiveness in vitro to a low concentration of ristocetin.
Additional screening tests for VWD include those that measure Factor VIII activity, VWF antigen (VWF:Ag), VWF binding to collagen (VWF:CB) and VWF ristocetin cofactor activity (VWF:RCo). Of particular interest herein is VWF:RCo, which is presently the standard for measurement of VWF function. VWF:RCo utilizes an ability of VWF to bind platelet GPIb following activation by ristocetin, which results in a VWF-dependent agglutination of platelets that can be measured quantitatively by platelet aggregometry or turbidometry. See, Macfarlane D, et al., “A method for assaying von Willebrand factor (ristocetin cofactor),” Thromb. Diath. Haemorrh. 34:306-308 (1975). In fact, an international reference standard for VWF:RCo was assigned a biologic activity in international units by the World Health Organization (WHO) and the Scientific and Standardization Committee of the International Society on Thrombosis and Haemostasis (ISTH).
Unfortunately, VWF:RCo, has several shortcomings. For one, VWF:RCo has high intra- and inter-assay imprecision because of its dependence on ristocetin. See, e.g., Chng W, et al., “Differential effect of the ABO blood group on von Willebrand factor collagen binding activity and ristocetin cofactor assay,” Blood Coagul. Fibrinolysis 16:75-78 (2005); Favaloro E, “An update on the von Willebrand factor collagen binding assay: 21 years of age and beyond adolescence but not yet a mature adult,” Semin. Thromb. Hemost. 33:727-744 (2007); and Riddel A, et al., “Use of the collagen-binding assay for von Willebrand factor in the analysis of type 2M von Willebrand disease: a comparison with the ristocetin cofactor assay,” Br. J. Haematol. 116:187-192 (2002). Federici et al recently described an alternative assay with improved reproducibility that used recombinant GPIb in an enzyme-linked immunosorbant assay of VWF binding; however, it is ristocetin dependent. See, Federici A, et al., “A sensitive ristocetin co-factor activity assay with recombinant glycoprotein Ib for diagnosis of patients with low von Willebrand factor levels,” Haematologica 89:77-85 (2004).
In addition, VWF:RCo does not always reflect the true in vivo function of VWF when mutations or polymorphisms are in the ristocetin-binding region of VWF. For example, some individuals have VWF mutations that show a reduced interaction with ristocetin such that VWF:RCo is markedly reduced (e.g., <0.12 IU/dL), although they have no bleeding symptoms even with a major surgical challenge. See, Flood V, et al., “Common VWF haplotypes in normal African-Americans and Caucasians recruited into the ZPMCB-VWD and their impact on VWF laboratory testing,” Blood 10:Abstract 714 (2007); Mackie I, et al., “Ristocetin-induced platelet agglutination in Afro-Caribbean and Caucasian people,” Br. J. Haematol. 50:171-173 (1982); and Miller C, et al., “Measurement of von Willebrand factor activity: relative effects of ABO blood type and race,” J. Thromb. Haemost. 1:2191-2197 (2003). These individuals, who appear to have a polymorphism in the ristocetin-binding region, do not have an abnormality in the binding of VWF to platelet GPIb.
Furthermore, VWF:RCo is affected by high-affinity VWF/platelet disorders. For example, individuals with platelet-type, pseudo VWD have GPIb mutations that cause spontaneous binding of their platelets to VWF. See, Franchini M, et al., “Clinical, laboratory and therapeutic aspects of platelet-type von Willebrand disease,” Int. J. Lab. Hematol. 30:91-94 (2008); Miller J & Castella A, “Platelet-type von Willebrand's disease: characterization of a new bleeding disorder,” Blood 60:790-794 (1982); and Miller J, “Platelet-type von Willebrand's Disease,” Thromb. Haemost. 75:865-869 (1996). Likewise, individuals with type 2B VWD have VWF mutations that cause spontaneous binding to platelets. See, Weiss H, “Type 2B von Willebrand disease and related disorders of patients with increased ristocetin-induced platelet aggregation: what they tell us about the role of von Willebrand factor in hemostasis,” J. Thromb. Haemost. 2:2055-2056 (2004).
Because of the wide variability and reproducibility of VWF:RCo, the art desires a VWF function assay that does not require a platelet aggregation agonist, such as ristocetin (i.e., ristocetinless).